Hexagonal boron nitride (hereinafter called “boron nitride”), because of having lubrication capability, high thermal conductivity, insulation capability and so on, are now widely used as releasing agents for solid lubricants, molten glasses and aluminum or the like as well as fillers for thermal radiation materials.
To be compatible with higher performances of recent computers and electronic hardware in particular, measures against thermal radiation have increased in importance, and attention has been directed to the high thermal conductivity of boron nitride.
In recent years, it has been studied to add boron nitride to the resin layers of printed wiring boards and flexible copper-clad laminated sheets for the purpose of imparting high thermal conductivity and insulation to them.
While generally available boron nitride has an average particle diameter of a few μm to 20 μm, some resin substrates for printed wiring boards and flexible copper-clad laminated sheets have a thickness of the order of several tens μm, and large average particle diameters of boron nitride result in poor dispersability in resins, failing to obtain smooth surfaces, or with that boron nitride dispersed, there are hard spots appearing, often making it impossible to keep the strength of the resin layer high. For these reasons, there is mounting demand for boron nitride fine particles of the submicron order (0.1 μm).
To have high thermal conductivity, the boron nitride must be of high purity (low total impurity or oxygen content in particular) and high crystallinity, and the same goes for boron nitrite fine particles of the submicron order (0.1 μm).
On the other hand, the boron nitride, because of its characteristic scaly shape, is less capable of dispersion in a resin.
To improve the dispersability of inorganic powders such as boron nitride in resins, surface treatment using a silane coupling agent or the like is usually effective.
However, such surface treatment was often ineffective for boron nitride because of the presence of surface functional groups on its end surface alone.
It follows that if thick, scaly, submicron-order boron nitride fine particles having a large end surface area are obtained, they will be preferable for addition to the aforesaid resin layer.
The boron nitride is generally obtained by reactions at high temperatures between a boron source (boric acid, borax, etc.) and a nitrogen source (urea, melamine, ammonia, etc.).
However, most of boron nitride obtained by this method aggregates into an average particle diameter of a few μm to 20 μm; in order to obtain boron nitride of the submicron order, it is required to prepare boron nitride by disintegration of boron nitride obtained by the aforesaid method or by a method different from the aforesaid method.
Referring how to disintegrate boron nitride, there is a report about disintegration using a jet mill or the like (Patent Publication 1).
With these methods, however, an active surface appearing during pulverization is so extremely susceptible to oxidation with the result that the obtained boron nitride fine particles will have a high total oxygen content.
A metal foil-clad sheet using boron nitride powders having improved dispersability has also been proposed in the art (Patent Publication 2). This patent publication refers to the use of boron nitride having an average primary particle diameter of 0.2 to 4 μm, an aspect ratio of 2 to 30 and an oxygen concentration of 0.1 to 1% by weight, but the specifically mentioned boron nitride has an aspect ratio of 7.3 or greater and an oxygen concentration of 0.25% by weight or higher. Patent Publication 2 says nothing about the graphitization index, and is silent about any boron nitride having an average particle diameter of 0.05 to 2.0 μm, a graphitization index of 3 or less, a total oxygen content of 0.20% by mass or less and an aspect ratio of 6.0 or less.
Referring to the preparation of boron nitride fine particles by a method different from the aforesaid one, there have been reports about how to obtain boron nitride fine particles by a gas-phase synthesis process (Patent Publications 3, 4 and 5).
However, boron nitride fine particles obtained by these methods, because of having low crystallinity and a high total oxygen content, is found to be less than satisfactory in terms of boron nitride's characteristics: lubrication capability and high thermal conductivity.